The invention relates to detecting bypassed hydrocarbons in subsurface formations.
Oil and gas production companies typically want to produce as much hydrocarbon as possible in each down-hole drilling operation. Quite often a well contains recoverable quantities of hydrocarbon gas in formations bypassed during drilling.
Pulsed neutron capture (PNC) tools have been used for years to evaluate the presence of hydrocarbon gas behind well casings in bypassed formations. PNC tools operate under the theory that neutrons generated by the tools and traveling with sufficient energy will interact with surrounding atoms to produce energy in at least two different ways. First, a high-energy neutron will collide "inelastically" with a nucleus, exciting the nucleus and causing the nucleus to release a gamma ray. Second, the same neutron eventually will lose enough energy that it will reach a "thermal" state and will be "captured" by another atomic nucleus, which in turn will release a gamma ray of capture.
Most PNC tools measure the thermal neutron capture characteristics, or macroscopic capture cross-section ("sigma" or ".SIGMA."), of a formation by detecting and counting gamma rays of the second type, i.e., those that occur as a result of thermal neutron capture, over a given period of time. In general, the presence of hydrocarbons in a formation increases the neutron capture time and therefore decreases sigma. In some formations, however, the measurement of sigma does not adequately identify certain forms of hydrocarbon trapped in the formation. For example, conventional sigma measurements often fail to identify natural gas in formations containing larger proportional amounts of shale suspended in a sand or thin shale laminations layering the sand, such as those found throughout the Gulf of Mexico. In these areas, natural gas often can be identified by detecting and counting only the number of gamma rays produced by inelastic collisions between the pulsed neutrons and atomic nuclei in the formation.
One PNC tool manufacturer produces a tool that attempts to detect bypassed gas reservoirs by observing the ratio of two different inelastic counts, one taken by a gamma ray detector located nearer the source of pulsed neutrons, and the other taken by a detector located farther from the neutron source. This ratio is measured at various depths to generate a "qualitative" graph, known as a RIN ("ratio of inelastics") curve. According to the developer of the RIN technique, the RIN curve provides a shallow measurement of hydrocarbon in the formation, i.e., two to four inches into the formation behind the well casing.